Saga of a volcano that erupted for too long

(ORDO NEWS) — On the afternoon of October 20, 2020, Icelandic Prime Minister Katrin Jakobsdouttir, speaking live with a Washington Post columnist, suddenly interrupted her monologue, said in surprise.

“God, an earthquake”, waited a few seconds, added that the house was still standing, apologized to the interlocutor and returned to the story of how her government is coping with the second wave of covid in the country. This 5.6 magnitude quake was the strongest in Reykjavik in twenty years.


Icelanders are not surprised by earthquakes: Iceland is a tectonically very active region.

If you ask any inhabitant of the country how many earthquakes he felt throughout his life, everyone will remember at least one day when the earth left from under his feet. The last major shock was two aftershocks in 2000 that passed magnitude 6.

It quickly became clear that this time the shocks were different. Normal Icelandic earthquakes are caused by the movement of plate blocks, and now researchers have immediately determined that these shocks are associated with magma making its way to the surface.

But no one could believe in a future eruption – the capital region was volcanically calm for as long as 800 years.

Earthquakes, which were more frequent and stronger than usual in the Reykjavik area, have been observed by seismologists from the Icelandic Meteorological Center since December 2019. Then immediately three of the six local volcanic systems were in the zone of seismic activity.

As it turned out later, the magmatic fluids with the melt were to blame for the growing shaking. They were pushed into the aquifers, which caused the rock layers to rise up a couple of centimeters. This swelling of the territory gave rise to a series of shocks that were picked up by seismographs.

Within a year after that, the magma fluids tried to break through to the surface four times, but did not succeed. The jolt that disturbed the Prime Minister and the people of Reykjavik was one of the magma’s attempts to make their way up.

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Map of earthquakes in the Reykjanes Peninsula from August 2019 to November 2020
The shocks associated with the first, second, and third episodes of magma-filled fractures are shown by green, red, and pink circles. Gray and purple colors indicate earthquakes that occurred before January 20 and after July 23, 2020, respectively. The light blue square is the position of the Geldingadalir volcano Yesim
Cubuk-Sabuncu et al

Since the end of February 2021, seismic activity on Reykjanes has become even more noticeable: in the next three weeks, the peninsula has already experienced tens of thousands of earthquakes. Their epicenters were concentrated around the Fagradalsfjall mountain range.

For the first time since the beginning of stationary observations, the region shook for so long without interruption.

The seismic crisis attracted the attention of researchers also because the hypocenters of shocks, the points of origin of earthquakes, consistently rose from the depths to the surface: excess pressure broke the fragile crust, and magma began to vigorously make its way up.

Chapter first. Childhood

March 19, 2021, 20:45 The magma has finally broken through the surface. A fissure opened in the Geldingadalir Valley, and for the first time in eight centuries, fresh lava flowed down Reykjanes. Thus began the eruption of Fagradalsfjall.

The location of the valley, close to the capital, allowed the installation of numerous surveillance devices, including cameras, gas sensors, rain and slag collectors. Lava began to flood the valley at a rate of five cubic meters per second.

Already in the first hours of the life of a newborn eruption, volcanologists from the University of Iceland delivered samples of solidified lava to the laboratory for X-ray spectral microanalysis.

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Selection of fresh lava for analysis. Hot rock is lowered into a bucket of cold water

“We expected the Fagradalsfjall eruption to be similar to previous events on the peninsula. But the first results came as a surprise,” says Simon Matthews, a geochemist at the University of Iceland, in an interview with N + 1.

The analysis showed that in the basalts that the volcano erupted, there is a lot of magnesium oxide – more than eight percent. Basalt is the main rock in Iceland. Of these, 90 percent of the island is composed.

The parental magma from which the Fagradalsfjadl lava was formed did not have time to “mature” in a reservoir in the earth’s crust, but got outside directly from the mantle, therefore such a rock is called primitive. Such melts are like young wine: you can see the primary (fresh) mantle component in them.

And while primitive magmas are not uncommon on Reykjanes, this time Fagradalsfjall scooped up more miscellaneous material from the mantle than before. Therefore, a couple of weeks after the start of the eruption, the composition of the lava changed dramatically.

Depleted melts (from the mantle, in which there are few incoherent elements that were present in it even before melting, in particular, rare-earth metals) were suddenly replaced by enriched ones (they contain a lot of incoherent elements).

To understand why there is so much magnesium oxide in the basalts from the new volcano, geologists decided to take a closer look at the site of the eruption. It turned out that the pressure at which the magma solidified was about 5.2–5.8 thousand atmospheres, that is, it happened at a depth of more than 14 kilometers.

The boundary of the earth’s crust and upper mantle under Reykjanes is located just at a depth of 15 kilometers. It turns out that the volcano took the magmatic melt almost from the mantle, near the Mohorovichic boundary, which separates the crust from the mantle.

In this way, the Fagradalsfjadl eruption differs from other volcanic events that Icelanders have witnessed in recent decades. Most of the eruptions originated in magma chambers under the central volcanoes.

These chambers are inside the earth’s crust, much closer to the surface than the upper mantle. For example, the Eyjafjallajökull explosion in 2010 was triggered by the injection of basalt into a pocket of acidic magma at a depth of five kilometers.

And in 2011, the volcano Grimsvötn poured out lava from a reservoir at a depth of less than three kilometers and melted the Vatnajökull glacier.

Iceland’s exuberant volcanism is caused by its position on the border of the North American and Eurasian plates, which are spreading at a rate of two centimeters per year.

This separation process is additionally superimposed by the work of the mantle plume.

This boundary position and plume interaction feeds Iceland’s 30 volcanic systems and produces regular eruptions, at least once every five years.

Fissure eruptions, in which lava flows out of an elongated fracture in the earth, were also different in the memory of the researchers. Since the settlement of the island, there have been 14 of them. “From 1975 to 1984, the Krapla volcano in the north of the island was episodically active.

Magma was formed as a result of partial melting of the mantle, accumulated in a shallow chamber in the earth’s crust, and then poured out through a crack,” explains Saimundur Halldorson. The previous fissure eruption, Holuhroin, was caught by the Icelanders in 2014-2015, which happened in the Baurdarbung volcanic system.

Then the magma reservoir, according to clinopyroxene barometry, was at a depth of eight kilometers – also much higher than the mantle. “Even though Holuhroin was fed from a deep pocket of magma, it was still in the Earth’s crust,” Saymundur says.

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Chapter two. Youth

April 2021. In the first week after the beginning of the eruption, the narrow magma conduit did not pass much melt, but due to the erosion of the walls it gradually expanded.

In April, a magma-filled crack dike broke through the surface three times, and the melt began to slosh out simultaneously from all six cones. At the end of April, all the outpourings again concentrated in one crater.

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May 1, 2021. Fagradalsfjall stopped for the first time. Hot lava stopped flowing from the main crater, but the researchers did not have time to announce the end of the eruption. A short lull was broken by a 200-meter fountain.

From that moment on, the volcano began to erupt in short cycles: it rested for three minutes, then splashed out huge walls of lava for five minutes, then gained strength again for three minutes.

The periodicity of the cycles was best recorded by seismographs. The instruments picked up waves in the range from one to five hertz, which arose due to the movement of magma through the supply channel.

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Real-time seismic amplitude measurements (RSAM) from 14 seismic stations on the Reykjanes Peninsula on the night of May 10-11, 2021

It is known that the reason for this remarkable behavior is the gaseous components of the magma inside the volcano. At high pressures, volatile substances are dissolved in magma, but as the melt rises to the surface, the pressure drops, and with it the solubility of volcanic gases.

Bubbles up to 10 meters in diameter begin to grow in the magma. At the surface of the lava pond, they burst and throw the melt into the air.

Over time, the pauses between emissions grew, and by the beginning of July they had increased to several hours and even days. “Ripple behavior depends on the top hundred meters of the magma conduit,” Thorvaldur Tourdarson, a professor at the University of Iceland, explained at the time.

In this part of it is a reservoir that has grown in size with the expansion of the channel. Now the eruption can be described by the cycle of filling-draining the reservoir. When magma fills the reservoir with magma, activity in the crater drops. When the tank is full, activity resumes, emptying the tank, and so on in a circle.

Magma chose a place for the eruption extremely well. The bark breach occurred in the central part of the uninhabited valley, so there was no need to evacuate the population and flocks of sheep.

There are no farms in Geldingadaler, the nearest town lies eight kilometers to the southwest. They decided not to close the international airport on Reykjanes – the crack did not throw out ash, therefore, unlike the Eyjafjallajökull eruption in 2010, volcanic particles did not threaten aircraft turbines.

May 22, 2021. Lava flowed from the “home” valley to the neighboring one, and the Icelanders began to take action.

In order to protect the nearby highway – an important transport artery of the peninsula – from being captured by flows, the municipal authorities decided to dig a channel for lava, direct the lava flow into it, and throw a bridge over it.

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Distribution of lava flows by October 2, 2021
Mounds marked with red rectangles

Around the same time, the first and only project to curb the eruption started. The Department of Civil Protection sent bulldozers to the lava flows, and four meters of embankments rose in the upper reaches of the Merardalir valley.

The erection of artificial barriers was an attempt by the authorities to redirect and slow down the spread of the flows. The embankments did not last very long: the very next day, the lava crossed over the obstacles and flowed to the coast. Then the authorities admitted that their experiment ended faster than expected.

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Chapter three. Maturity

While the lava peacefully seized the territory, the gases thrown into the atmosphere covered Reykjanes. The areas northwest of the eruption suffered the most: in the cities of Njardvik and Vogar, there was too much sulfur dioxide in the air.

The sensors recorded more than 1000 micrograms of SO2 per cubic meter – which is already dangerous for people with respiratory diseases.

The “healthy” limit is 350 micrograms per cubic metre, which is why the Icelandic Environment Agency has requested that babies not be left in strollers for naps and that outdoor activities be limited.

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Satellite image of the Sentinel gas emissions from the Fagradalsfjall eruption

June 1, 2021. Every third inhabitant of the island had already managed to go to the crack at least once by the beginning of summer, even despite the gas curtain. It seemed that all the attention of the country was focused on the eruption.

Employees of the Ministry of Emergency Situations were on duty around the clock and shifted the paths for new lava flows.

Volcanologists calculated the range of lava spray and determined the safe radius for observing the eruption. The head of disaster monitoring even helped find a place among the fresh lava flows for a local band to perform.

The Icelandic meteorological office regularly posted recommendations for volcanic walks based on weather forecast and air quality. One restriction has remained unchanged since March: not to take dogs with you.

Sulfur dioxide builds up in depressions, and pets can inhale this toxic gas because their heads are closer to the ground. And water from puddles near the volcano is dangerous for tetrapods because of the fluorine dissolved in it.

August 30, 2021. Five months after the start of the first eruption in the central part of the valley, the thickness of lava flows exceeded 80 meters, and the volume of lava flowing out of the volcano crossed the mark of 140 million cubic meters.

But the pauses between episodes of activity were getting longer and longer, and the eruptions themselves were getting weaker.

“We must remember that a cubic meter of basalt weighs three tons. Magma has to make its way through such a heavy thickness. This certainly affects the strength of the eruption,” Helga Torfadouttir, a volcanologist at the University of Iceland, explained to N+1.

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September 2, 2021. The eruption stopped once again. But nine days later, a seismic station near Mount Fagradalsfjall again recorded a growing tremor, and at night new hot streams flowed over the hardened black lava crusts.

Since the beginning of the 20th century, only three eruptions on the island have lasted longer than Fagradalsfjadl in 2021, and for Reykjanes, this is generally the first breakthrough in hundreds of years.

The last period of volcanic activity on the peninsula was more than 800 years ago and ended in 1240. Then the melt at different times flowed out of more than a dozen volcanic fissures for 400 years.

The Svinahroin lava field reminds of those burning times. It is known as “Christian lava” because, a couple of tens of kilometers from the gaping crack, the Alfingian parliament decided whether the Icelanders should become Christians.

October 18, 2021. The Icelandic meteorological center has downgraded the volcanic hazard code from orange (little ash eruption) to yellow (high seas).

December 18, 2021. The authorities announced the end of the Fagradalsfjall eruption: for three months, fresh lava did not appear on the surface.

Chapter Four. Prophecy

Of course, studies of the features of this eruption, unique for Iceland, did not stop with the end of lava emissions.

In September 2022, Saimundur Halldorson and colleagues published an article in Nature where they collected the results of geochemical analyzes and explained why the eruption was so long and multi-staged.

“If you look at the previous fissure eruption on the island, Holuhroin in 2014-2015, you can see that it is very uniform,” says Saimundur, an assistant professor at the University of Iceland. “During the time of Holuhruin, a land mass comparable to Manhattan was formed.

You can select a rock anywhere in this “Manhattan” and the set of its rock-forming elements will be the same.” With Fagradalsfjall the situation is different.

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The ratio of the content of potassium oxide and titanium oxide in the rocks in the first 30 days of the eruption

In the first three weeks alone, the composition of the Fagradalsfjadl lava changed dramatically. The content ratios of K2O/TiO2 oxides and La/Yb rare-earth metals approximately doubled, reflecting changes in the mantle component.

In addition, concentrations of incompatible trace elements also increased in the lava, confirming that the volcano begins to supply melt from an enriched mantle source over time. “This is the first time we have seen such a shift in composition when the source of the basalt is at a very deep depth.

The fracture is fed by a geochemically heterogeneous reservoir in the upper mantle. It looks like the volcano is pulling very different portions of magma out of the mantle,” Simon Matthews explained these results.

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The content of oxides of magnesium, potassium and titanium in igneous rocks from March 19 to August 13

In the lava from the beginning of the eruption, there were few rare earth elements. Such depleted lava came from a relatively depleted mantle source at a depth of 15–16 kilometers.

However, already in April, the melts became more and more enriched in incoherent elements, which corresponded to a deeper and more enriched mantle source. By the beginning of May, “deep” magma already dominated.

This dynamic is different from classic textbook examples. Usually, the magma reservoir is filled slowly, and before the eruption, melts of different compositions have time to mix well. Volcanologists do not expect sharp jumps in the chemical composition of lava during an eruption.

For example, the Kilauea volcano in Hawaii may erupt for months or years, but the geochemistry of its rocks remains almost unchanged. During the Fagradalsfja eruption, the lava composition changed more in a month,

A new Icelandic volcano has given geologists the first opportunity to see in real time how magma of very different composition mixes in lava during an eruption. It turned out that a new portion of magma from the upper mantle can get into the reservoir in just 20 days and mix with the magma that is already in it.

Because of this, the lavas of the first three weeks turned out to be geochemically much more diverse than all that erupted during the last volcanic period on Reykjanes in the 9th-13th centuries AD.

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How the food source of Fagradalsfjadl changed in 2021: from depleted melts (yellow) to deeper and enriched ones (orange)

Volcanologists found out how deep the magma reservoir of the new volcano was, what processes took place in it before and during the eruption, calculated the flow of lava and measured the height of the fountain in the crater.

But the main question: how this volcano works – they have not answered. Not every day there are events that radically change the understanding of familiar things. Thanks to the eruption of Fagradalsfjad, such a revelation came. Scientists will have to think about its meaning.


August 3, 2022. The Reykjanes eruption has resumed. A new fissure opened in the neighborhood of the one that appeared a year and a half earlier. She froze, however, much faster, after 18 days. This cessation of activity marks a new phase of volcanic-tectonic unrest in Reykjanes.

The Icelandic meteorological center is not ready to predict whether seismic activity will resume in the near future. But it looks like the people of Reykjavik need to get used to the glow on the horizon.


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